This invention relates to an efficient process for the sintering of silicon carbide articles in a plasma gas atmosphere. Silicon carbide articles, produced in accordance with the invention, have superior qualities to prior art sintered articles.
Silicon carbide has several physical and chemical properties which make it an excellent material for high temperature, structural uses. Mechanically, silicon carbide is a hard, rigid, brittle solid which does not yield to applied stresses even at temperatures approaching its decomposition temperature. Because of its high thermal conductivity, silicon carbide is an excellent material for heat exchangers, muffle type furnaces, crucibles, gas-turbine engines and retorts in the carbothermic production and distillation of zinc Silicon carbide is also used in electrical resistance elements, ceramic tiles, boilers, around tapping holes, in heat treating, annealing and forging furnaces, in gas producers, and in other places where strength at high temperatures, shock resistance and slag resistance are required. Properties associated with silicon carbide refractory and ceramic materials are superior strength, high elastic modulus, high fracture toughness, corrosion resistance, abrasion resistance, thermal resistance, and low specific gravity.
Prior art sintering processes for silicon carbide, in general, tend to be inefficient and slow. Long furnace retention times are necessary when using conventional sintering processes, which results in poor energy utilization, excessive furnace gas consumption and high maintenance costs.
Many ceramic or refractory materials are sintered in prior art tunnel or periodic kilns which are fired by energy released from the combustion of fossil fuels with air or oxygen. If the ceramic or refractory material can be exposed to air and/or the products of combustion, then the kiln may be directly fired, in which case, the heating and utilization of energy may be reasonably efficient. However, sintering of silicon carbide should be performed in the absence of oxygen or oxygen-bearing gases, including water and carbon dioxide, to prevent formation of oxides, which may result in products having undesirable physical and chemical properties. Under such conditions, fossil fuel-fired furnaces may be used but the ceramic or refractory materials must be kept in a controlled environment, such as a retort, isolated from the combustion products of the fuel. Such heating is indirect, inefficient and slow. On a commercial scale, an apparatus such as a tunnel kiln requires about 70-90 hours (including the cooling cycle) to sinter silicon carbide refractory or ceramic materials.
Prior art electric kilns are more commonly used to sinter alpha silicon carbide ceramic or refractory articles under controlled atmospheres, but again tend to be energy inefficient and slow. In the case of a kiln equipped with graphite heating elements, the voltage can be controlled and the kiln can be heated to fairly high temperatures, yet there are several disadvantages: 1) The heating elements have a limited size, complex shape and must be kept under a strictly controlled atmosphere to maintain a long life; and 2) Furnace size is limited and it is difficult to achieve a uniform temperature in this type of kiln because the heating elements provide only radiant heat. Because of radiant heat transfer, as well as a heat element size limit, the kiln has a poor load density, a limited productivity and a poor energy efficiency. A typical sintering cycle time using a prior art electric kiln is about 24 hours (including cooling).
Plasma arc technology has recently been applied to the production of refractory and ceramic materials to reduce the furnace energy requirements and retention times. Plasma sintering of refractory and ceramic articles results in higher density and higher strength products than those made by conventional prior art processes.
Plasma arc fired gases differ greatly from ordinary furnace heated gases in that they become ionized and contain electrically charged particles capable of transferring electricity and heat; or, as in the case of nitrogen, become dissociated and highly reactive. For example, nitrogen plasma gas dissociates into a highly reactive mixture of N.sub.2 -molecules, N-atoms, N.sup.+ -ions and electrons. This dissociation or ionization greatly increases the reaction rates for sintering ceramic or refractory articles. Nitrogen, for example, which dissociates at about 5000.degree. C. and one atmosphere pressure, would not dissociate under the normal furnace sintering conditions of about 1500.degree. C.-2000.degree. C. Thus, the use of plasma gases results in a highly reactive environment, which greatly increases the reaction sintering rate.
Plasma arc technology has generally only been used for the fusion of high temperature materials and not for sintering or reaction sintering. This is because the required sintering temperature for most ceramic or refractory materials is usually less than 2500.degree. C., whereas the average temperature of gases heated with a plasma arc torch is above 4000.degree. C. At such high temperatures, the refractory or ceramic materials may decompose. For example, U.S. Pat. No. 3,432,296, entitled "Plasma Sintering," to McKinnon et al, discloses a process for sintering refractory oxide materials at temperatures of less than 1650.degree. C. using radio frequency electromagnetic energy to generate the plasma gas.
However, a plasma gas can be superheated to effect ionization or dissociation, while the ceramic or refractory material is then directly heated by this preheated gas to a much lower temperature. For example, nitrogen plasma gas heated to about 3000.degree. C. will bring silicon carbide refractory articles up to a temperature of 1000.degree. C.-1600.degree. C. in two to eight hours; and nitrogen plasma gas heated to about 4000.degree. C. will bring the articles up to a temperature of 1900.degree. C.-2200.degree. C. in the same time period. Thus, a plasma gas may be heated to a much higher temperature than the sintering temperature required, depending on furnace geometry, plasma input power and load density.